The existing sewing machines usually include an upper shaft which drives, through a needle bar driving linkage assembly, one needle bar to move up and down. The needle bar, during the up and down movement, is penetrated through needle holes on a needle plate, so that cloth is sewn by the coordination between the upper threads and the base threads mounted on a base bobbin. With higher requirements of sewing, upper threads of different colors are sometimes required to sew the cloth. Thus, manual change of threads is required during the sewing. This operation is complex, resulting in low efficiency of sewing.
Chinese Utility Model Application No. 201120342948, titled Needle Bar of Two-Needle Sewing Machine and Switching Mechanism Thereof disclosed a needle bar of a two-needle sewing machine and a switching mechanism thereof. This needle bar comprises a hollow needle bar body, the lower end of which is fixedly connected to a needle bar head used for mounting a needle. A reset spring, a first wedge, a second wedge and a switching compression lever are sequentially provided inside the needle bar body. The upper end of the switching compression lever is exposed from the needle bar body. The tip of the first wedge faces downward while the tip of the second wedge faces upward. A first through hole is provided at a position, corresponding to the first wedge, on the side wall of the needle bar body, and a first slider is provided inside the first through hole; and a second through hole is provided at a position, corresponding to the second wedge, on the side wall of the needle bar body, and a second slider is provided inside the second through hole. The switching compression lever comprises a needle bar, a needle bar frame, a needle bar sleeve, a needle bar driving mechanism and a switching compression block, the needle bar sleeve being connected to the needle bar driving mechanism, the needle bar frame being connected to a translation device. The two needle bars in this utility model may be penetrated by two upper threads of different colors. Therefore, a pattern with two colors may be sewn at one time, avoiding changing the threads manually. However, such a two-needle bar structure is complex and has high requirement on the machining precision, resulting in high manufacturing cost. Moreover, manual change of threads is still required when sewing a pattern of three or more colors.